Planning - how important is it to create mostly quads and few triangles?
I'm no expert on mesh topology, but I do know that good topology (with nicely flowing edge loops and almost only quads) is important especially if you want to animate and deform your mesh, and less so in most other cases.

Some tools work better with quads than tris, such as the subdivision modifier which sometimes generates weird artefacts, especially when the mesh is irregular. EDIT: Rigging and skinning is also easier when you have a clean quad based topology with nice flowing edge loops.

In most other cases triangles will pose no problem (I might have forgotten a few other examples for issues, which others here might add, but there probably aren't that many). You can texture, shade, light, 3D print with ease even if your entire mesh is made of tris.